1. Field of the Invention
This application relates generally to optical devices utilizing photonic-bandgap fibers.
2. Description of the Related Art
Photonic-crystal fibers have been the subject of much interest and developments in recent years. (See, e.g., J. Broeng et al., “Photonic crystal fibers. A new class of optical waveguides,” Optical Fiber Technology, Vol. 5, pages 305-330 (1999); J. C. Knight et al. “Photonic crystals as optical fibers-physics and applications,” Optical Materials, Vol. 11, pages 143-151 (1999); R. S. Windeler et al., “Silica-air microstructured fibers: Properties and applications,” Optical Fiber Communications conference, San Diego, Calif. (1999).) Because of their unique properties, including low optical nonlinearities and good temperature stability, hollow-core photonic-bandgap fibers (PBFs) are finding interesting applications as sensors and delivery systems. (See, e.g., V. Dangui et al., “Phase sensitivity to temperature of the fundamental mode in air-guiding photonic-band gap fibers,” Optics Express, Vol. 13, pages 6669-6684 (2005); H. K. Kim et al. “Fiber-optic gyroscope using an air-core photonic-bandgap fiber,” Proceeding of SPIE—The International Society for Optical Engineering, 17th International Conference on Optical Fibre Sensor, OFS-17, Vol. 5855, pages 198-200 (2003).) In addition, the propagation loss in hollow-core PBFs is not limited by the core material, and it is expected that the propagation loss can be exceedingly low. The hollow core can be filled with air, or other gases or combinations of gases to generate the desired light-matter interaction. With further research and improvements, hollow-core PBFs could well replace conventional fibers in optical communication links.
One of the most important components of fiber circuits for these applications is the optical fiber coupler. Fiber circuits utilizing hollow-core PBFs can be readily assembled using conventional (i.e., solid-core) fiber couplers, which can be either butt-coupled or spliced to the hollow-core PBF. However, this approach suffers from various shortcomings. Butt-coupled junctions often do not provide sufficient mechanical stability, and splices of dissimilar fibers can introduce significant amount of back-reflection and associated loss, as well as being somewhat difficult to fabricate. In addition, the use of a conventional fiber coupler introduces a length of solid-core fiber in the hollow-core fiber circuit, thereby re-introducing dispersion and nonlinearity into the fiber circuit and negating some of the benefits of using the hollow-core PBFs.
Examples of applications in which these effects can be detrimental include, but are not limited to, delivery by a hollow-core PBF of pulse-distortion-free high-peak-power pulses for fluorescence imaging (see, e.g., T. P. Hansen et al., “All-fiber chirped pulse amplification using highly-dispersive air-core photonic bandgap fiber,” Optics Express, Vol., 11, pages 2832-2837 (2003)) and in hollow-core PBF gyroscopes (see, e.g., R. A. Bergh et al., “Single-mode Fibre Optic Directional Coupler,” Electronics Letters, Vol. 16, pages 260-261 (1980); J. V. Wright, “Variational Analysis of Fused Tapered Couplers,” Electronics Letters, Vol. 21, pages 1064-1065 (1985).), where the Kerr effect is advantageously minimized and additional lengths of solid-core fibers are to be avoided.